AVCHD is a highly compressed video formats and usually recorded by camcorders like Panasonic, Canon, Sony, JVC, Hitachi, etc. As to Final Cut Pro X, it is a professional non-linear editing software application and offers non-linear, non-destructive editing of any Quick Time formats including DV, HDV, MPEG-4, MOV, etc. But FCP X does not support all camcorders AVCHD files. So sometimes you still encounter can't import AVCHD to Final Cut Pro X issues:*

How to convert AVCHD to Final Cut Pro X*friendly format*on Mac (macOS Sierra, El Capitan Yosemite, Mavericks, Lion included). And in order to keep the excellent quality of the files, it employs a professional*Pavtube AVCHD to FCP X Converter for Mac*(Windows version) to be the assistant. Moreover, this powerful program has preset for Final Cut Pro, so that you can easily choose the FCP format - Apple ProRes MOV. (Read Review)*